It is known in the prior art to control the speed of a vehicle propulsion direct current series motor by a chopper apparatus including a thyristor switch device in series with the motor. Speed control of the motor is provided by varying the width of the voltage pulses supplied to the motor such that the resulting average power supplied to the motor establishes the operational speed thereof. A commutation circuit including a charge capacitor has been used to provide a biased voltage across a conducting thyristor device for commutating the conduction of that thyristor device. A traction motor is operative in a motoring mode when the passenger vehicle is being propelled or accelerated along a track and in a brake mode when the vehicle is being stopped or decelerated.
It is known in the prior art to control the operation of one or more transit vehicle motors with a chopper apparatus as described in U.S. Pat. Nos. 3,559,009, of J. M. Mills, 3,543,121 of L. G. Miller and 3,535,503 of H. C. Appelo et al.
As described in a published Article in the Westinghouse Engineer for March 1973 at pp. 34-41 the average voltage supplied to the motor armature is controlled by adjusting the ratio of chopper OFF-time to the chopper ON-time with the resulting average motor armature current determining the motor torque for moving the vehicle along a track. In the motoring or power mode of operation the motors of a vehicle are connected by mechanical switches in relation to a direct current voltage source such that current is supplied through a chopper when the chopper is ON and through the motors to ground. When the chopper is turned OFF, the energy stored in the motor reactor and the inductance of the motor field maintains current flow in the motor circuit through a free-wheeling diode. In the brake or deceleration mode of operation, the motors in the prior art were reconnected by mechanical changeover switches with the motors operative as self-excited generators to provide dynamic or regenerative braking of the vehicle. With the chopper ON, the motor current increases and with the chopper OFF, the current is forced into the power supply through the free-wheeling diode by the motor reactor.
It is known in the prior art as shown by U.S. Pat. No. 4,095,153 of T. C. Matty et al. to utilize a microprocessor chopper control of regenerative brake current buildup by providing an upper current limit to control chopper-ON operation and a lower current limit to control chopper-OFF operation.